Aqueous solutions of acrylamide polymers (i.e., "polyacrylamides" or "PAMs") are useful in applications such as thickening and dewatering of industrial and sewage wastes and the clarification, i.e., by flocculation techniques, of turbid aqueous solutions. In addition, as discussed for example in U.S. Pat. No. 4,034,809 to Phillips et al., these materials have also been found to be useful in secondary and tertiary oil recovery processes wherein the polymer solution is introduced into subterranean oil-bearing formations to help increase the amount of crude oil recovered.
Typically, PAMs are available as powders or as finely divided solids and are dissolved in water to form aqueous solutions for use in a particular application. It is difficult and time-consuming, however, to dissolve these dry polymers in water, particularly in the case of hydrolyzed acrylamide polymers. This is because the dry polymer is not readily dispersable in the liquid and thus has a tendency to form lumps upon contact with the water. Such lumps typically require an extended period, sometimes as long as 6-10 hours, to dissolve.
U.S. Pat. No. 3,624,019 to Anderson et al. discloses a polymeric latex comprising a water-in-oil emulsion having dispersed therein finely divided droplets of a water-soluble vinyl addition polymer solution. Polyacrylamide is a preferred polymer disclosed by the reference. The polymer-containing emulsions described by Anderson et al. are stable. When inverted in the presence of water, the polymer goes into solution in a very short period of time as compared to the extended, i.e., 6-8 hour, period required for the dissolution of a dry solid polymer as described above. A water-in-oil polymerization process and method for making latex polymers is described in U.S. Pat. No. 3,284,393 to Vanderhoff et al.
A major problem associated with acrylamide polymer lattices of the type described above, however, is that when they are hydrolyzed, the latex becomes unstable. The polymer present within the latex coagulates and precipitates out of the water-in-oil emulsion, thus producing a product with little or no commercial value.
U.S. Pat. No. 4,171,296 to Connelly et al. discloses a method for hydrolyzing acrylamide polymers within a polymer latex, comprising a water-in-oil emulsion containing finely divided polyacrylamide, wherein the latex is stabilized by the addition of an alkali stable organic surfactant prior to hydrolysis. The method for producing the acrylamide polymer emulsion comprises polymerizing the acrylamide monomer utilizing an azo catalyst, i.e., 2,2'-azobis (isobutyronitrile) and a sequential monomer addition process. The disclosure of this reference is also limited to the production of polymers ranging in molecular weight from about 10,000 to 25,000,000.
Polymers such as those disclosed in Connelly et al., i.e., with a molecular weight up to about 25 million, are known to be relatively insensitive to detrimental side reactions. Such reactions have more of a tendency to occur in ultra high molecular weight PAMs. That is, the higher the molecular weight of the polymer, the greater the propensity for the polymer to become cross linked. These side reactions are known to lead to decreased performance by the polymer. Thus one skilled in the art would try to avoid them by the use of low to medium molecular weight acrylamide polymers.
There has therefore been a long felt need by those working in this field for partially hydrolyzed high and ultra high molecular weight acrylamide polymers for use in thickening and flocculating applications. Thus, an effective, efficient method for hydrolyzing high molecular weight acrylamide polymer emulsions, while reducing or eliminating entirely the degree of side chain reactions, to form, respectively, high and ultra high molecular weight polymers, is both desirable and needed. Ultra high molecular weight polymers are particularly useful and are thus of even greater value due to their capacity to more effectively flocculate solids.